Method for calibrating a downhole receiver used in electromagnetic instrumentation for detecting an underground conductor

ABSTRACT

A method utilized with an underground conductor detection system for calibrating a downhole transmitter to compensate for detuning of the transmitter antenna by geologic formations. The method comprises sending a synchronization signal to the transmitter antenna, measuring the current flow through the transmitter antenna and adjusting the current to a constant level, and measuring the phase difference between the transmitter antenna current and the synchronization signal. A receiver transmitter is calibrated by sending a synchronization signal to a radiating antenna in the receiver that in turn sends a calibration signal to the receiver antenna that is directed over the entire signal pathway back to surface located signal processing equipment. Another method of underground conductor detection sends a surface wave to the downhole receiver to cancel the effect of the surface wave modulation on a target wave being radiated by the underground detector. Another method of underground conductor detection utilizes a receiver tuned loop antenna oriented orthogonal to the magnetic dipole of the transmitter antenna for discriminating against reception of a primary wave. Another method of detecting anomalous geological zones such as tunnels, utilizes a low to medium frequency tomographic scan to cancel the effect of geological conductivity noise in a high to very high frequency tomographic scan of a region suspected of containing the anomalous geological zone.

This is a divisional of co-pending application Ser. No. 07/734,302,filed Jul. 19, 1991, now U.S. Pat. No. 5,185,578, issued Feb. 9, 1993,which is a divisional of Ser. No. 07/466,494, filed Jan. 17, 1990, nowU.S. Pat. No. 5,066,917, issued Nov. 19, 1991.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an improved method andapparatus for detecting underground electrical conductors surrounded byless electrically conducting rock and more particularly to a method fordetecting are veins or electrically conducting equipment located inunderground tunnels or boreholes.

2. Description of the Prior Art

Several techniques are currently used in military operations to detectunderground tunnels. These include visual observation of surfacefeatures, surface drilling, use of accoustical and seismic systems andthe deployment of various surface and downhole electromagnetic (EM) wavepropagation methods.

Of these techniques, the EM techniques are the most promising becausethey are less sensitive to munition firings and can be made lesssensitive to random geologic structure noise. Two EM techniquespresently in use are a cross-hole high frequency diffraction detectionmethod claimed in U.S. Pat. No. 4,161,687, issued to Lytle, et al. and across-hole pulsed EM system (PEMSS-II) developed by the SouthwestResearch Institute and currently used by the U.S. and South KoreanArmies.

R. J. Lytle, et al., in "Cross-borehole Electromagnetic Probing toLocate High Contrast Anomalies"; Geophysics, Vol. 44, pp. 1667-1676(October 1979), discloses a theoretical basis for detecting tunnels bydiffraction scanning.

All of these cross-hole techniques are designed to detect changes in theelectrical parameters of the geologic medium caused by the tunnelintersection with a vertical plane between two drillholes. However,these techniques utilize downhole transmitting and receiving antennasthat are connected to surface equipment by electrically conductivecable. This use of electrically conductive cable interferes withmagnitude and phase shift measurements and prevents the reliable use ofsynchronous detection techniques.

In general, use of antennas and electromagnetic wave propagation methodsin slightly conducting natural rock for remote sensing and mapping ofsubsurface geologic features, for applications in hardened militarycommunications, and radio communications with miners working or trappedin underground tunnels has been reported in the literature. The subjectarea has been investigated for communications with submerged submarines.Review papers, Hansen, R. C., "Radiation and Reception with Buried andSubmerged Antennas," IEEE Trans. on Ant. and Prop.; May 1963; and Moore,R. K., "Effects of Surrounding Conducting Medium on Antenna Analysis",IEEE Trans. on Ant. and Prop.; May 1963, trace the historicaldevelopment of the canonical theory from its late 18th century beginningwith Heaviside, O., "Electrical Papers", Vols. I and II, MacMillan andCompany, Ltd., London, England 1882. The theoretical problem consideredthe interaction of antennas and EM field components with slightlyconducting geologic medium. For radio communications, the problemconsiders radio wave propagation along the surface of the earth, directpaths through the earth, up over and down paths between submarines, andthe possibility of a deeply buried natural waveguide in the earth. Forgeological investigations, the problem considers the detection of halosof chemically mineralized ore zones associated with faults and dikes,sandstone layers and voids in limestone that trap oil and gas, seams ofcoal, trona, potash, and anomalies that interfere with orderlyextraction of valuable resources. Sommerfield, A., "Uber die Austreitungder Wallen in der Drathlosen Telegraphic", Ann. Physik, Ser 4 Vol. 81,No. 17, pp. 1135-1153, Dec. 1926, provided early theoretical insite intosurface wave communications, and Wait, J. R. (guest editor) May 1963issue of IEEE Trans. Ant. and Prop. vol. AP. 1, No. 3, contributedknowledge regarding communications and techniques for investigatingsubsurface geological features.

J. R. Wait and D. A. Hill, "Coaxial and Bifilar Modes on a TransmissionLine in a Circular Tunnel", Preliminary Report to U.S. Bureau of Mineson Contract No. H0122061 (September 1974); relates to an investigationof propagation of guided waves in tunnels and formulated a theoreticalmodel showing that monofilar and bifilar propagation modes exist fortwo-wire cable and trolley tracks and power cable types of conductors.

Also, a method for measuring the bulk electrical parameters of a regionof the earth which involves measuring the intensity and phase shiftvalues of the magnetic field of an electromagnetic wave simultaneouslyreceived in two boreholes is described in R. N. Grubb, P. L. Orswell andJ. H. Taylor, "Borehole Measurements of Conductivity and DielectricConstant in the 300 kHz to 25 MHz Frequency Range", Radio Science, Vol.II, No. 4 (April 1976).

J. R. Wait, "The Magnetic Dipole Antenna Immersed in a ConductingMedium", Proceeding of the IRE (Oct. 1952), points out that afundamentally different power dissipation relationship exists betweenelectric and magnetic dipole antennas. In the electric dipole case, theradial wave impedance near the dipole is largely real, whereas theimpedance is imaginary in the case of the magnetic dipole. The largereal impedance results in more energy dissipated near the electricdipole than flows out to large distances.

R. F. Harrington, "Time Harmonic Electromagnetic Fields", McGraw Hill,N.Y. (1961), describes a formula for calculating the current flowproduced in a conductor by an incident electric field.

Synchronous detection principles are described by W. R. Bennett and J.R. Davey in "Data Transmission", McGraw Hill Book Company (1965).

Philip F. Panter, in "Modulation, Noise and Spectral Analysis Applied toInformation Transmission", McGraw-Hill Book Co., pp. 461-503 (1965),describes the application of receiver frequency feedback compressiontechniques to decrease the occupied bandwidth of frequency or phasemodulated radio signals.

Finally, U.S. Pat. No. 4,577,153, "Continuous Wave Medium FrequencySignal Transmission Survey Procedures for Imaging Structures in CoalSeams", by L. G. Stolarczyk describes a method for constructing imagesof structures in coal seams using the radio imaging method.

SUMMARY OF THE PRESENT INVENTION

It is therefore an object of the present invention to describe a methodfor discriminating against the primary wave in a buried conductordetection system.

It is another object of the present invention to present a method forcalibrating the transmitting and receiving antennas in a buriedconductor detection system.

It is another object of the present invention to provide a method forreducing phase jitter in the transmitted and received signals.

It is another object of the present invention to present a method forusing surface radio waves to detect buried conductors.

It is another object of the present invention to present a method forcombining transmitter and receiver antennas for buried conductordetection in the same borehole.

It is another object of the present invention to present a method foreliminating geological noise from a buried conductor tomographic scan.

Briefly, a preferred embodiment of the present invention includes animproved method for detecting underground electrical conductors. Theprior art methodology for detecting buried conductors included the stepsof generating a first electromagnetic field with a transmitting antenna;using the electric field component of the first electromagnetic field toinduce a synchronized current flow in an underground electricalconductor; using the magnetic field component of a secondelectromagnetic field (scattered wave), generated by the current inducedin the electrical conductor, to induce a signal in an antenna of a phasecoherent receiver deployed in a drillhole; and using synchronousdetection measurememts and analysis to confirm the existence of theelectrical conductor.

In the present invention, the loop of a downhole receiving antenna ispositioned orthogonally to the loop coils of a downhole ferrite rodtransmitting antenna. This orientation of the antennas discriminatesagainst the reception of the primary wave by the receiver, thusenhancing reception of the scattered wave. This orientation of thereceiver antennas is useful both when the buried conductor is containedin a tunnel, e.g. a trolly track or telephone or power line, and whenthe buried conductor is a geological formation such as a rock mass (e.g.a massive sulfide stringer or a skan in an ore body) or a coal seam.

In another embodiment of the present invention, the receiving antenna iscalibrated (synchronously) by using an untuned broadband antennacalibration circuit in the receiver, such as a long wire or an untunedloop or rod antenna, to generate a calibration signal at the receiveroperating frequency. The calibration signal establishes the magnitudeand phase of the receiver transfer function along the receiver signalpath to a synchronous detector. The transmitting antenna is calibratedby measuring (controlling) the magnitude and phase of current flowing inthe loop antenna. A phase comparator is used to compare the phase of thetransmitting antenna current to the phase of the system synchronizationsignal.

Phase jitter in the phase locked loop (PLL) circuits contained in thedownhole transmitter and receiver is minimized by using fast rise timewaveform signals as the synchronization signals that are sent over thefiber optic cables to the downhole transmitter and receiver.

In another embodiment of the present invention, the transmitting antennais eliminated and surface radio waves propagating in theearth/ionosphere waveguide are utilized to excite the buried conductor.A surface receiving unit is utilized to generate a signal that is sentto the downhole receiver to cancel the phase or frequency modulation ofthe surface wave. The signal received by the downhole receiver isconverted to a continuous wave signal that is processed using real timesynchronous (autocorrelation) techniques.

In another embodiment of the present invention a ferrite rodtransmitting antenna is connected to a rotatable elongated loop receiverantenna for insertion in a single borehole. The receiver antenna can berotated so as to permit reception of either a primary wave or asecondary wave.

In another embodiment of the present invention a tomographic scan of atunnel that is free of geological noise is generated by subtracting theresults of a low to medium frequency data scan from the results of highor very high frequency data scan on a pixel by pixel basis.

An advantage of the present invention is that a method is presented forenhancing reception of a scattered wave at a downhole receiver.

Another advantage of the present invention is that a method is presentedfor calibrating a downhole transmitter and receiver.

Another advantage of the present invention is that phase jitter in thetransmitter and receiver are minimized by sending fast rise time digitalsynchronization signals to the downhole transmitter and receiver.

Another advantage of the present invention is that a method is presentedfor utilizing surface waves in the detection of buried conductors.

Another advantage of the present invention is that a method is presentedfor utilizing the surface wave to cancel the phase shift or frequencymodulation in the downhole receiver.

Another advantage of the present invention is that a method fordetecting a buried conductor using a transmitter and a receiverpositioned in one borehole is presented.

Another advantage of the present invention is that a method is presentedfor eliminating geological noise from a buried conductor tomographicscan.

These and other objects and advantages of the present invention will nodoubt become obvious to those of ordinary skill in the art after havingread the following detailed description of the preferred embodimentswhich are illustrated in the various drawing figures.

IN THE DRAWINGS

FIG. 1 is an elevational, partially sectioned view of a geological areawith an underground tunnel and showing the drillhole configurationaccording to method I of the prior art;

FIG. 2 is a top elevational view of an alternative embodiment of anantenna configuration for use with method I of the prior art;

FIG. 3 is a top elevational view of another alternative embodiment of anantenna configuration for use with method I of the prior art;

FIG. 4 is a block diagram of the apparatus used in phase synchronoussignal transmission and phase coherent signal reception according to theprior art;

FIG. 5 is a block diagram of the synchronous detector which is acomponent of the apparatus shown in FIG. 4;

FIG. 6 is an elevational, partially sectioned view of a geological areawith an underground tunnel and showing the drillhole configurationaccording to method II of the prior art;

FIG. 7 illustrates the electromagnetic wave field components produced bycurrent flow in a tuned loop antenna;

FIG. 8 is an elevational, partially sectioned view of a geological areawith an underground tunnel and showing the drillhole configurationaccording to method III of the prior art;

FIG. 9 is an elevational, partially sectioned view of a geological areawith an underground tunnel and showing the drillhole configurationaccording to method IV of the prior art;

FIG. 10 is an elevational, partially sectioned view of a geological areawith a vertically oriented underground electrical conductor showing thedrillhole configuration according to method V of the prior art;

FIG. 11 is a block diagram of an alternative embodiment of the apparatusshown in FIG. 4;

FIG. 12 is an elevational, partially sectional view of an ore veincontaining a plurality of drillholes separated by a distance to bedetermined according to method VI of the prior art;

FIG. 13 is a schematic diagram of an orthogonal receiver/transmitterantenna system according to the present invention;

FIG. 14 shows a system for detecting the scattered wave from aconductive surface according to the present invention;

FIG. 15 is a block diagram of a system for calibrating a downholereceiver and a downhole transmitter according to the present invention;

FIG. 16 is an elevational, partially sectioned view of a geological areawith an underground tunnel and showing a receiver configuration forusing a surface wave to detect buried conductors according to thepresent invention;

FIG. 17 is a block diagram of a receiving system for use with thereceiver configuration of FIG. 16;

FIG. 18 is an elevational, partially sectioned view of a drillholecontaining a buried conductor detection system containing a transmitterand a receiver according to the present invention;

FIG. 19 is an elevational, partially section view of a buried conductordetection system for use with a geological background subtraction methodaccording to the present invention; and

FIG. 20 is a graphical representation of a fast rise time waveform.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1-12 show buried conductor detection systems of the prior art.

FIG. 1 shows an underground tunnel 10 surrounded by a rock layer 12 andcontaining a plurality of electrical conductors 14, illustrated in theform of train tracks. The rock layer 12 conducts electricity much lessefficiently than the electrical conductors 14. The electrical conductors14 may be any objects contained in the tunnel 10 which conductelectricity and which extend along a length of the tunnel 10. Theelectrical conductors 14 could also be an electrically conducting objectembedded in the rock layer 12 such as a thin mineralized conducting orevein. Other examples of objects that could function as the electricalconductor 14 include small diameter copper power or telephone cables;metal air pipe; trolley power conductor; electrolytic water flowing inplastic water pipes within tunnel 10; electrolyte water filling theentire tunnel 10; seepage of water from dams; or plumes of toxic wastefrom waste containment repositories.

A plurality of drillholes 18 extend downward through the rock layer 12from a terrestrial surface area 20. A transmitter 24 is located on thesurface area 20 and is coupled by a loop antenna 25 to at least onecable 26 (a grounded dipole). A coupler cable 28 electrically connectsthe transmitter 24 to the loop antenna 25. When the transmitter 24 isturned on, current flowing in the loop antenna 25 (loop current) isphase synchronized with a signal applied to an input terminal 30 of thetransmitter 24. By induction, the loop current causes synchronizedcurrent to flow in the cable 26. The cable 26 functions as a long linecurrent antenna and can be a relatively straight piece of insulated wirehaving a length in the range of 100 to 1500 feet. Alternatively, aplurality of cables 26 may be deployed in a fan-like pattern, as shownin FIG. 2, which subtends an angle θ of approximately ninety degrees.The cable 26 could also be formed into a cable loop having a largediameter X as shown in FIG. 3.

Returning to FIG. 1, a coherent frequency source unit 32 is opticallyconnected to the transmitter 24 by a fiber optic cable 34 running fromthe unit 32 to the input terminal 30. A receiver 36 is also opticallyconnected to the coherent frequency source unit 32 by a fiber opticcable 37 comprising an uplink fiber optic fiber 38 and a downlink fiberoptic fiber 40 (shown in FIG. 4). The receiver 36 includes anelectrically short ferrite vertical loop magnetic dipole antenna 41 withthe loop coils of antenna 41 approximately located in the x-z plane(magnetic moment vector aligned with the z-axis). The coherent frequencysource unit 32 is capable of generating at least two low frequencysignals in the frequency range of one to three hundred kilohertz (1-300kHz). The coherent frequency source can also generate frequencies in thehigh frequency (HF) and very high frequency (VHF) bands for use in theprecise measurement of distance. The receiver 36 is deployed in at leastone of the drillholes 18 at a depth d₁.

The transmitter 24, the coherent frequency source unit 32 and thereceiver 36 may comprise the components shown in FIG. 4. A quartzcrystal reference oscillator 42 generates a crystal oscillator outputsignal S_(c) having a crystal frequency f_(c) which is divided in adivision state machine 44 by an integer K to produce a first referencesignal S_(refa) having a reference frequency f_(ref) and a phase of zerodegrees. All signals leaving the coherent frequency source unit 32 arephase synchronized with the signal S_(refa). The division machine 44also produces a second reference signal S_(refa) offset in phase fromS_(refa) by ninety degrees. The signals S_(refa) and S_(refb) arerequired in the synchronous detection process.

The signal S_(refa) is used in a conventional phase lock loop circuit 46to generate a preliminary transmit signal S_(T) ' which is phasesynchronized with the signal S_(refa). The oscillator 42 output signalS_(c) is also used as a preliminary receiver local oscillator signalS_(Lo) '. Before the signals S_(T) ' and S_(Lo) ' are sent to thetransmitter 24 and receiver 36 respectively, they are converted infrequency to the required system operating frequency band. The signalS_(T) ' is mixed by difference mixing in a mixer 48 connected to aquartz crystal conversion oscillator 50 to produce an operating transmitsignal S_(T). Similarly, the signal S_(Lo) ' is mixed with the samequartz crystal conversion oscillator 50 signal in a mixer 52 to producean operating receiver local oscillator signal S_(Lo). After mixing, thesignals S_(T) and S_(Lo) have the frequencies f_(o) and f_(Lo),respectively. When the signals S_(Lo) and S_(T) are derived by thissignal generation process, the signals are said to be phase coherent.The phase drift and incidental phase variation occurring in theconversion oscillator 50 appears identically in the total phase of thesignals S_(T) and S_(Lo). The phase drift and incidental phasevariations are cancelled in the receiver mixing process.

The signal S_(Lo) is sent by the downlink fiber optic fiber 40 to amixer 60 contained in the receiver 36. The receiver 36 also comprisesthe ferrite vertical loop magnetic dipole antenna 41 which iselectrically connected to an amplifier 64 which is electricallyconnected to the mixer 60. The mixer 60 is electrically connected to anIF amplifier/optical fiber transmitter unit 66 which is connected by theuplink fiber optic fiber 38 to a synchronous detector 70 contained inthe source unit 32. A microcomputer 72 is electrically connected to thedetector 70. The reference signal S_(refa) is sent to the phase lockloop circuit over the lead 76 and to the synchronous detector 70 overthe lead 78. The reference signal S_(refb) is sent to the synchronousdetector over the lead 80.

In the coherent frequency source unit 32, the crystal frequency f_(c) ofthe signal S_(c) could be 10.24 MHz, the reference frequency f_(ref) ofthe signal S_(refa) could be 2.5 kHz and the integer K could be 4096.

Alternatively, the synchronization signal S_(refa) can be sent over afiber optic cable to a phase lock loop (PLL) circuit in each downholeinstrument (see FIG. 11). The PPL's generate the transmit signal (S_(T))and the receiver local oscillation signal (S_(Lo)) in the downholeinstruments.

The antenna 41 is capable of receiving electromagnetic signals. Forexample, for a properly oriented magnetic field H.sub.θ at 100 kHz, theloop emf is given by:

    emf=(4.02×10.sup.-2)H.sub.θ.

The loop signal is amplified by the amplifier 64 and mixed with thesignal S_(Lo) in the mixer 60. The frequency S_(Lo) is given by:

    f.sub.Lo =f.sub.o -f.sub.ref.

Difference mixing in the mixer 60 produces an intermediate signal S_(IF)that is represented by the form:

    S.sub.IF =B sin (2πf.sub.ref+θ.sub.2).

The phase θ₂ is the sum of all phase shifts encountered in the signalpath commencing at the source unit 32 output terminal to the outputterminal of the IF amplifier 66. The geologic medium phase shiftcontribution θ_(M) is included in θ₂. The conventional synchronousdetector 70 along with the microcomputer 72 measure the magnitude andphase signal S_(IF).

FIG. 5 is a block diagram of the synchronous detector 70. The signalS_(IF) enters the detector 70 through the uplink fiber 38 where itencounters a pair of analog switch units 90 and 92 which receive thesignals S_(refa) and S_(refb), respectively. The switches 90 and 92perform the multiplication processes of S_(IF) ×S_(refa) and S_(IF)×S_(refb). After low pass filtering by a pair of low pass filters 94 and96, a pair of orthogonal voltages e_(x) and e_(y) are generated and fedto a multiplexer (analog-to-digital converter) 98. The phase θ₂ iscalculated as

    θ.sub.2 =tan.sup.-1 e.sub.x /e.sub.y

and the amplitude B is calculated as

    β=(e.sub.x.sup.2 +e.sub.y.sup.2).sup.1/2.

The theory of operation of the system shown in FIg. 1 is as follows.First, the source unit 32 generates a phase synchronizing signalrequired for the signal S_(T) of the frequency f_(o) which is sent tothe transmitter 24 via the cable 34. The signal S_(T) may be a sine wavehaving the form Asin (2πf_(o) t+θ_(A)) where A is the amplitude, t istime and θ_(A) is the phase shift value. The transmitter 24 and theantenna 25 excite primary current flow in the cable 26. The primarycurrent flow causes a first electromagnetic field EM₁ to propagate downthrough the rock layer 12. If an electric field component E_(z) of thefield EM₁ encounters one of the electrical conductors 14 orientedparallel to the length of cable 26, a strong secondary current flow willbe induced in the parallel conductor 14. Secondary current flow willalso be induced in conductors 14 that are not oriented parallel to cable26, but the magnitude of this nonparallel current flow will be small.The fan-like pattern of cables 26, shown in FIG. 2, and the largediameter loop configuration, shown in FIG. 3, are designed to maximizethe possibility that the polarized electric field component E_(z) willencounter a parallel electrical conductor 14.

The secondary current flow will propagate along the conductor 14 andwill generate a second electromagnetic field EM₂ which will propagatethrough the rock layer 12 with a frequency f_(o), but with a differentamplitude and phase shift than that of signal S_(T). The verticalmagnetic dipole antenna 41 within the receiver 36 is properly orientedto receive the magnetic component H.sub.φ of EM₂ as a received signalS_(R). The signal S_(R), has a wave form Bsin (2πf_(o) +θ_(B)) where Bis the new amplitude and θ_(B) is the new phase shift value. The signalS_(R) is amplified by the amplifier 64 and sent to the mixer 60. Themixer 60 is being supplied with the signal S_(Lo) generated in acoherent frequency source unit 32 and transmitted to the mixer 60 viathe fiber 40. The signal S_(Lo) has the form Dsin 2π(f_(o) -f_(ref))twhere D is the amplitude of the signal S_(Lo). The mixer 60 forms afinal signal S_(IF) by adding S_(Lo) to S_(r) yielding equation (1).

    S.sub.IF =C sin(2πf.sub.ref t+θ.sub.m)            (1)

where C is the amplitude of S_(IF) and θ_(m) is the phase shift.

The final signal S_(IF) is sent through the uplink fiber 38 to thesynchronous detector 70 where the amplitude C and the phase shift θ_(m)are determined by using synchronous detection principles. In the absenceof a conductor 14, the amplitude C will be zero because the separationbetween the receiver 36 and the cable 26 is too great to permitreception of the signal S_(T).

The cable 26 is located at a specific position and the receiver 36 ismoved between the plurality of drillholes 18 with at least one phaseshift measurement taken at each drillhole 18 location. Taking aplurality of amplitude and phase shift measurements at a differentplurality of depths d₁ within each drillhole 18 would improve theprobability of detecting the conductor 14. Alternatively, the receiver36 could be held within a single drillhole 18 and the position of thecable 26 could be varied.

For convenience, the method of placing the cable 26 at a surfacelocation and detecting the conductor 14 by making measurements with thedownhole receiver 36, will be referred to as method I. The feasibilityof method I has been verified by a combination of theoretical andempirical measurements.

In an actual experiment, a 100 meter long cable was positioned on thesurface of the York Canyon Mine in the general direction of the man andmaterial passageway located approximately one hundred meters below thesurface. The mudstone roof rock had a conductivity of approximately1×10⁻² mhos/meter. The cable was excited with approximately 100 mA ofcurrent at 300 kHz. A current of eleven microamperes was measured in atelephone cable located in the passageway.

Table A shows the minimum detectable secondary current levels fornon-coherent and coherent phase receivers when the conductor 14 and thereceiver 36 were separated by thirty and seventy meter radial distances.These figures indicate that the eleven microamperes signal actuallymeasured in the York Canyon Mine is sufficient for detecting theconductor 14 located seventy meters from the receiver 36 using thesynchronous detection technique of method I.

                  TABLE A                                                         ______________________________________                                        Minimum Detectable Tunnel Electrical Conductor                                Secondary Current in Microamperes                                             (f = 100 kHz; σ = 10 × E - 03 mhos/m; ε.sub.r = 10)                   Coherent          Non-Coherent                                    Antenna     Receiver          Receiver                                        Diameter    30 m   70 m       30 m 70 m                                       ______________________________________                                        1 inch      0.52   1.78       50.1 177.8                                      2 inches    0.13   0.44       12.58                                                                              48.08                                      ______________________________________                                    

The data in Table A was calculated by the following method. First, thefollowing equation (2) was used to calculate the magnetic fieldcomponent H.sub.φ produced by the secondary electromagnetic field EM₂.##EQU1## where π=the radial distance from the conductor;

I_(s) =the secondary current in amperes; and

H₁.sup.(2) (κρ)=the Hankel function of the second kind of first order.

(Equation 2 is taken from M. L. Burrows, "ELF Communications Antennas"Peter Peregrinus Ltd., England (1978)).

Next, when Hφ threads the loop area of the ferrite vertical magneticdipole antenna 41, a receiving antenna response, given by Faraday's law,is produced according to:

    emf=ANμ.sub.r ω|H.sub.φ |   (3)

where

A=the area of the loop antenna in square meters,

N=number of turns,

μr=the magnetic permeability of the antenna, and

ω=the radian frequency of the system.

The sensitivity of a non-phase coherent RIM receiver is better than tennanovolts and the phase coherent sensitivity improves to 0.1 nanovolt. Avertical ferrite rod antenna exhibits the electrical properties shown inTable B.

                  TABLE B                                                         ______________________________________                                        Ferrite Rod Antenna Electrical Properties at 100 kHz                          Diameter    Area (m.sup.2)                                                                           emf (volt)                                             ______________________________________                                        1 inch      5.07 × 10.sup.-4                                                                   (4.02 × 10.sup.-2)H.sub..0.                      2 inches    2.03 × 10.sup.-3                                                                   (1.62 × 10.sup.-1)H.sub..0.                      ______________________________________                                         (H.sub..0.  = the value of the magnetic field at the receiver location)  

Finally, the minimum detectable current flow in the tunnel electricalconductors is determined from Table B data, the threshold sensitivity ofthe downhole receivers and the value of magnetic field determined fromEquation (2) at radial distances of thirty and seventy meters. It may bepossible to increase the receiving loop response by using laminatedsignal transformer metal, e.g. nickel-iron alloy of the Permalloy type.Very high permeabilities of the order of 10⁴ times the free space valuecan be obtained. Building the core cross-section area from insulatedlaminations can increase the area and reduce core loss to the pointwhere they are negligible compared with the winding loss. The long andslender antenna will achieve good coupling to the signal field withoperating frequencies below ten kHz; however, these low frequencyantennas may also be sensitive to the earth's geomagnetic field. Sincethe vertical orientation of the antenna will be used in the borehole,the antenna will not strongly couple to the earth's field.

FIG. 6 shows an alternative method for detecting the electricalconductor 14'. For convenience, this method will be referred to asmethod II. Elements in FIG. 6 that are identical to elements describedwith respect to FIG. 1 are designated by the same reference numeral usedin FIG. 1 followed by a prime symbol.

In FIG. 6, a synchronized downhole transmitter 100 is positioned at adepth d₂ within one of the drillholes 18' and is optically connected tothe source unit 32' by a fiber optic cable 102. The transmitter 100includes an electrically short vertical magnetic dipole antenna 104 suchas a ferrite rod antenna with the loop coils of the antenna 104 orientedapproximately in the x-z plane.

FIG. 7 shows the fields produced by a magnetic dipole aligned with the yaxis. The cartesian coordinate system (x, y, z) is oriented so the looplies in the horizontal x-z plane with its vertical magnetic moment(M=NIA) aligned along the y axis. Hence, FIG. 7 depicts a verticalmagnetic dipole antenna. The spherical coordinate system (θ, φ, -r) isused to describe the general orientation of field components in thegeologic medium 12. A meridian plane 106 is orthogonal to the x-z planeand includes the y axis. The magnetic dipole field components are givenby the following equations 4 through 6.

Meridian Plane Aximuthal Component ##EQU2## where κ=β-iα The electricalvector is perpendicular to the meridian plane and subscribes concentriccircles around the y axis magnetic dipole moment vector. The terms inthe equations 4, 5 and 6 have been arranged in the inverse powers of r.In the immediate neighborhood of the magnetic dipole moment, the"static" and "induction" fields in 1/r³ and 1/r² predominate while atdistance r>>λ/2π or kr>>1 only the "radiation" field has significantvalue. The radiation fields are given by the following equations 7 and8: ##EQU3## The radiation fields are transverse (orthogonal) which isexpected of wave propagation at great distances from all electromagneticsources. The magnitude of the magnetic field component H.sub.θ can beexpressed in terms of the ratio α/β and β_(r) as ##EQU4## where, κ=β-iαand

α=attenuation constant (imaginary part of wave number) in neper/meter;and

β=phase constant (real part of the complex wave number in radians/meter)

The phase θ is mathematically represented by ##EQU5##

The axis of the receiving loop antenna 104 is always parallel to theaxis of the drillhole 18'. The loop response is given by Equation 3(Faraday's Law).

The processing of the received signal S_(R) in the source of unit 32'recovers the logarithm of the loop response as ##EQU6## When βr is lessthan 0.5 radian, the magnetic field is relatively independent of themedia electric parameters; however, when βr is greater than 2, theamplitude of the field is highly dependent on the ratio (α/β). If r isless than 0.5 radian, the phase shift change with range is less thanfour degrees. When βr is between 0.5 and 1.5 radians, the phase shiftchange with distance may increase or decrease with range and depends on(α/β). Above βr=1.5, the phase shift increase with range. Phase shiftdepends strongly on (α/β).

The magnitude of the electric field component E can be expressed as##EQU7## and the phase θ by ##EQU8##

In a uniform geologic medium, the meridian plane magnetic field(H.sub.θ) component is polarized normal to the area of the receivingloop antenna 41' when θ=π/2. At 100 kHz, the loop emf is given by

    emf=(4.02×10.sup.-2)H.sub.θ.                   (14)

Returning to FIG. 6, method II can be summarized as follows: The sourceunit 32' sends the signal S_(T) having the frequency f_(o) to thetransmitter 100. The signal S_(T) has the same waveform as waspreviously described for method I, specifically, A sin (2πf_(o) t+θA).The transmitter 100 then causes a first electromagnetic field EM₁, topropagate through the rock layer 12'. As in method I, secondary currentwill be induced in the conductor 14' when an electric field componentE_(z) of the field EM₁ encounters one of the electrical conductors 14'oriented in the z direction. Calculations have shown that currentresponse increases in the frequency range of 1 to 500 kHz. Thus, anyfrequency in this band could be used as the frequency f_(o). However,since the receiver antenna 62 output voltage increases with frequency,it is advisable to use the highest practical frequency and 300 kHz isused preferrentially. The receiver 36', positioned in a separatedrillhole 18' from the transmitter 100, responds to the magnetic fieldH.sub.θ produced by the secondary current flow. The phase shift andamplitude values can be calculated from the data received at thereceiver 36' in the same manner as previously described for method I.

In method II, optimal search strategy will be achieved when thetransmitter and receiver drillholes 18' each have center linesorthogonal to the longitudinal dimension of the conductor 14', i.e.,when the conductor 14' and the drillholes 18' all lie along parallellines. Additionally, in method II, the drillholes 18' should be in closeproximity to the conductor 14', generally, within approximately 100meters. However, the drillhole 18' containing the transmitter 100 mustbe far enough away from the drillhole 18' containing the receiver 36' sothat the primary wave (EM₁) is extinquished at receiver 36'.Furthermore, a plurality of measurements can be taken at each drillhole18' location by varying the depth d₁ of the receiver 36' or the depth ofd₂ of the transmitter 100. The depths d₁ and d₂ can be different.

Table C lists the results of experimental measurements of the secondarycurrent flow induced in a two foot wide mine ventillation pipe(conductor 14') by the downhole transmitter 100. Magnetic field strengthmeasurements were made in the tunnel with a tuned loop antenna (300 kHz)and a field strength meter. The emf, defined by Equation (15) wasmeasured and recorded in decibels above one nanovolt.

                  TABLE C                                                         ______________________________________                                        Measured Field Strength Inside a Tunnel                                       (dB re 1 nanovolt)                                                                                Field        Field                                        Mea-  Approximate   Strength (H) Strength (H)                                 suring                                                                              Distance (ft) From                                                                          Center       Near                                         Station                                                                             Transmitter 44                                                                              of Tunnel (10')                                                                            Conductor (14')                              ______________________________________                                        1     300            85.6        100                                          2     250            82.0        100                                          3     200            86.0        101                                          4     150           101.0         96                                          5     100           111.0        103                                          6      50           123.0        *                                            7      50           131.0        *                                            ______________________________________                                         * = Pipe ends                                                            

    emf=AμN2πf|H|                      (15)

where

N=the number of turns used in the antenna design,

A=the area of the loop in square meters,

f=the operating frequency,

μ=magnetic permeability of the antenna, and

H=field strength from Table C.

FIG. 8 shows another alternative method for detecting the undergroundconductor 14'. For convenience, this method will be referred to asmethod III. Elements in FIG. 8 that are identical to elements in FIGS. 1or 6 are designated by the same reference numeral followed by a primedesignation.

In FIG. 8, the transmitter 100' and the receiver 36' are positioned inthe pair of drillholes 18' separated by a distance D, that straddle theconductor 14' (i.e., one drillhole 18' is located on each side of theconductor 14' so that the transmitter 100' and the receiver 36' arecontained in a vertical plane that bisects the conductor 14'). In methodIII, the maximum value for the separation distance D is approximatelytwenty meters. In one technique for using method III, referred to as areconnaissance scan, the transmitter 100' and the receiver 36' arealways maintained at the same depth, depth d₃ for example, with respectto each other. After a measurement is taken, the transmitter 100' andthe receiver 36' are each moved to a second depth, d₄ for example, andanother measurement is taken.

This cross-hole reconnaissance scan process is designed to measure thereceiver drillhole total field amplitude and the phase shift as both ofthese parameters vary with depth. The variation occurs at the depthcorresponding to the location of the conductor 14'. The transmitter 100'emits the plane wave signal S_(T) having the frequency f_(o) asdescribed previously for method II. The electric field component of thesignal S_(T) (E_(z) ^(i)) is polarized in the direction parallel to theconductor 14' (the z direction). When the signal S_(T) encounters theconductor 14' a scattered wave is produced which simultaneously producessecondary current flow in the conductor. This scattering phenomenaincreases with decreasing thickness of the conductor 14' when theelectric field component is z polarized. The electric field component ofthe scattered wave (E_(z) ^(s)) is also polarized in the z direction.The total electric field (E_(z)) at any point outside the conductor 14'is given by

    E.sub.z =E.sub.z.sup.i +E.sub.z.sup.s.                     (16)

Variations in the amplitude and phase shift parameters are determined bymeasuring E_(z) at the position of the receiver 36'. The phase shift andamplitude determinations are made as previously described for method I.For computational purposes, the receiver drillhole field is the vectorsum of E_(z) ^(i), computed at the radial distance r₁ extending from thetransmitter loop antenna 104' to the receiver loop antenna 41', andE_(z) ^(s) computed at a radial distance r₂ extending from the conductor14' to the receiver loop antenna 41'. The magnetic field componentsassociated with the E_(z) field are measured in the receiver.

In a second technique utilizing the method III transmitter/receiverconfiguration, the receiver drillhole field can also be measured versusthe depth of the receiver 36' above and below the depth of the conductor14'. In this technique, called a shadow scattering wave scan, thetransmitter 100' is held at a specific depth, d₄ for example, and thereceiver 36' is moved to incremental elevations, such as d₃, d₄ and d₅,above and below the conductor 14'. The magnetic field componentsassociated with E_(z) are measured at each incremental elevation.Alternatively, the depth of the receiver 36' could be held constant andthe depth of the transmitter 100' could be varied.

Table D lists the results obtained in a typical shadow scattering wavescan. In this system, f_(o) was 10 kHz and the conductivity of the rocklayer 12' was 0.001 mho/m. The distance D was twenty meters and theantenna 104' was five meters from the conductor 14'. With synchronousdetection techniques, amplitude resolution of better than 0.1 dB ispossible and phase shift resolution of better than 0.3 degrees can bemeasured.

                  TABLE D                                                         ______________________________________                                        Elevation Change                                                              Above/Below                                                                              Initial   Change In Initial                                                                              Phase                                   Conductor  Amplitude Amplitude Phase  Shift                                   (In meters)                                                                              (dB)      (dB)      (degrees)                                                                            (degrees)                               ______________________________________                                         0         65        5         44     88                                      13         65        4.5       44     84                                      20         65        3.9       44     75                                      ______________________________________                                    

Because of the limitation imposed on the method III technique by therestricted range of acceptable D values, method III is most useful forpinpointing the location of the conductor 14' after its rough locationhas been determined by methods I or II. Method III is also useful wherethe conductor 14' is a thin conducting ore vein contained in the rocklayer 12'.

FIG. 9 illustrates another alternative method for detecting theelectrical conductor 14'. For convenience, this method will be referredto as method IV. Elements of FIG. 9 identical to elements described withrespect to FIGS. 1, 6 and 8 are designated by the same reference numeralused in FIGS. 1, 6 and 8, followed by a prime symbol.

In FIG. 9, the receiver 36' is lowered into the drillhole 18'. Thereceiver 36' is connected to the source unit 32' by the cable 37'. Atleast one piece of mine electrical equipment 110 is located inside themine 10'. The mine electrical equipment 110 could be an electricalmotor, a trolley power system, a high energy transformer or any otherpiece of electrical equipment that produces non-continuous voltage orcurrent (electrical noise) in the electrical power distribution system.

In method IV, the mine electrical equipment 110 generates electricalnoise which induces current flow in the electrical conductors 14'. Thisconductor flow produces an electromagnetic field that propagates throughthe rock layer 12' where it can be detected by the receiver 36'. Forexample, electrical motors and power systems switching transients induceelectrical noise signal current flow in the underground mine electricalconductors. Switching transients occuring on surface power transmissionlines or in the underground mine produce multiple high energy transientsin the millisecond time duration range. AC induction motors producetriangular wave form currents during the motor startup period. Theresulting frequency spectrum exhibits a 1/f² amplitude function withspectrum components separated by the power system frequency. Trolleypower systems produce noise signal currents with a sinx/x amplitudefunction. A feature of this spectrum is that nulls in the sinx/xspectrum are relatively electrical noise free. High energy transformersfrequently produce ferroelectric response at 1800 Hz. These noisesignals produce current flow in the mine electrical conductors.

The methods I through IV can also be used in conjunction with eachother. For example, method IV could be used as a rough indicator of theexistence of the electrical conductor 14. Methods I or II could be usedto identify the approximate location of the electrical conductor 14 andmethod III could be used to pinpoint the location. Method III is bestsuited for detecting a thin conductive ore vein that intersects theplane between the drillholes 18.

FIG. 10 illustrates another method of the prior art. For convenience,this method will be referred to as method V. Elements of FIG. 10identical to elements described with respect to FIGS. 1, 6, 8 and 9 aredesignated by the same reference numeral used in FIGS. 1, 6, 8 and 9followed by a prime symbol.

In FIG. 10, an electrical conductor 114 extends vertically downward (inthe y-direction) from a surface region 116. In FIG. 10 the conductor 114is depicted as being a borehole circumscribed by a casing 118 made of ametal or some other material. Alternatively, the conductor 114 could bean uncased borehole or a vertical shear zone filled with highlyconductive mineralized rock or sea water or any other electricallyconducting object oriented in the vertical direction. The conductor 114need not extend all the way up to the surface region 116. The region 116may comprise, for example, land, concrete or conductive water. Theconductive water may fill the borehole.

A plurality of horizontal drillholes 120 extend horizontally away froman underground area 124 (i.e. approximately in the x-z plane). Anintermediate orientation is possible provided that the transmitted Efield is polarized with the conductor. The underground area 124 issimilar to the underground tunnel 10, but may or may not contain theplurality of electrical conductors 14. At least two of the drillholes120 should be in a horizontal plane (x-z plane) which is approximatelyperpendicular to the length of the conductor 114. These two drillholes120 are separated by the distance D and straddle the conductor 114.

The transmitter 100' is inserted into at least one of the drillholes120. However, in method V, the transmitter 100' includes an electricallyshort horizontal magnetic dipole antenna 126, such as a ferrite rodantenna, instead of the vertical magnetic dipole antenna 104'. The coilsof the antenna 126 lie approximately in the y-z plane of FIG. 10.

The receiver 36' is inserted into at least one of the drillholes 120 notcontaining the transmitter 100'. The receiver 36' includes anelectrically short horizontal magnetic dipole antenna 128, such as aferrite rod antenna, with coils that lie approximately in the y-z planeof FIG. 10.

The transmitter 100' and the receiver 36' are connected to the coherentfrequency source unit 32' by the fiber optic cables 102' and 37',respectively. In method V, the source unit 32' can be located in theunderground area 124.

Method V can be used to detect the vertical electrical conductor 114 inan analogous manner to that used in method III described previously(illustrated in FIG. 8). The existence of the conductor 114 can bedetected by a reconnaissance scan, where the transmitter 100' and thereceiver 36' are always maintained at parallel positions within thedrillholes 120 or by the shadow scattering technique where the relativeposition of the receiver 36' is varied incrementally with respect to theposition of the transmitter 100'.

In both techniques, total field amplitude and phase shift are measuredas was discussed previously in connection with method III. However, inmethod V, the electric field component of the signal S_(T) is polarizedin the y direction because that is the direction in which the conductor114 is oriented. An intermediate orientation is possible provided thatthe transmitted E field is polarized with the electrical conductor.

FIG. 11 shows an alternative embodiment of the coherent frequency sourceunit 32 represented by the general reference numeral 134. Elements inthe alternative embodiment 134 which are identical to the elements ofthe coherent frequency source unit 32 are represented by the samenumeral followed by a prime symbol.

In the unit 134, the phase lock loop circuit 46 is eliminated and areceiver phase lock loop (PPL) circuit 136 is connected between themixer 60' and the division state machine 44' by a fiber optic cable 138.In this configuration, S_(refa) =S_(Lo). The PPL circuit 136 is includedwithin the receiver 36' for insertion in the drillhole 18. A transmitterphase lock loop (PLL) circuit 140 is connected to the cable 138 by fiberoptic cable 142. The PPL circuit 140 is also connected to thetransmitter 100'. The PPL circuit 140, the transmitter 100' and theantenna 104' comprise a transmitter unit 142 which can be inserted inthe drillhole 18'.

With the unit 134, the audio frequency band S_(refa) can be sent overthe cables 138 and 142 to synchronize the PLL circuits 136 and 140respectively in each downhole probe. The fiber optic path bandwidth canbe less than 10 kHz resulting in lower cost design.

FIG. 12 illustrates another method according to the prior art. Forconvenience, this method will be referred to as method VI. Elements ofFIG. 12 which are identical to elements described previously withrespect to FIGS. 1, 6, 8, 9 and 10 are designated by the same referencenumeral used in FIGS. 1, 6, 8, 9 and 10 followed by a prime symbol.

In FIG. 12, the transmitter 100' and the antenna 104' have been loweredinto a vertical drillhole 150 along a centerline 152 which is a linecoincident with the geometric center of the drillhole 150. The drillhole150 is drilled vertically from an undercut level 154 through an ore vein156 to a sub-level 158. The ore vein 156 is a natural resource mediumwhich is to be mined by a technique such as block caving, verticalcrater, retreat or by the stope mining method. The undercut level 154 isa region cut on top of the vein and the sub-level 158 is a region cutunder the vein which runs approximately parallel to the undercut level154. The receiver 36' and the antenna 41' have been lowered into asecond vertical drillhole 160 which is approximately parallel to thedrillhole 150 and which has a centerline 162 coincident with thegeometric center of the drillhole 160. The centerlines 152 and 162 areseparated at various depths d₁ and d₂ by the distances D₁ and D₂,respectively. If the drillholes 150 and 160 are not exactly parallel, D₁will not be equal to D₂.

Method VI is used to determine whether the drillholes 150 and 160 areparallel or not. Whether the drillholes are parallel is importantbecause in the blockcaving, vertical crater, retreat or stope miningtechniques, the drillholes 150 and 160 are plugged and then loaded withan explosive. Detonation of the explosive cause fragmentation of the orevein 156 producing small size rocks (muck) which is removed byscoop-tram or long-haul-dump (LHD) equipment on the sub-level 158.Horizontal or near vertical slip planes can cause the drillholes 150 and160 to have an inclination azimuth angle that is different than zero. Ifthe crosshole distance D₁ or D₂ is too great, the fragmentation produceslarge boulders that increase mining costs. Therefore, measurement ofcrosshole distance can improve extraction efficiency.

In method VI, the transmitter 100' is used to launch an electromagnetic(EM) wave which propagates through the ore vein 156 to the receiver 36'.The intensity and phase of the EM field components depend on thedistance D₁ and on the electric parameters of the geologic medium suchas the electrical conductivity (σ), the dielectric constant (ε) and themagnetic permeability (μ). In method VI, the preferred operatingfrequency is 10 MHz and in some applications could be as high at 100MHz.

It can be analytically shown that the phase changes by 1.8 electricaldegrees for each inch of change in distance between the center lines 152and 162. A reference parameter such as the phase R' can be measured atthe depth d₁ corresponding the the distance D₁. At the depth d₁, thedistance D₁, can be accurately determined by a certified near surfacedownhole survey procedure such as surface laser survey instruments thatlocate center lines each drillhole. As the transmitter 100' and thereceiver 36' are simultaneously lowered to the new depth, d₂, theantenna 41' responds to the magnetic field component of EM wave. Thesource unit 32' reads and records the reference parameters, such as theintensity and phase of the received signal, as was described previouslyfor method I. Any phase shift change from the reference O_(R) indicatesa change in the drillhole centerline distance.

Method VI could also be used to detect changes in the centerlinedistance between horizontal drillholes by use of thereceiver/transmitter configuration shown in FIG. 10.

Various improvements of methods I-VI enhance detection of the electricalconductors 14 (or 14'). These improvements may be grouped into sixcategories: First, the receiver antennas may include a horizontalmagnetic dipole antenna to cancel reception of the primary wave and thusincrease the reception sensitivity of the scattered wave. Second, thetransmitters 100 or 100' and the receivers 36 or 36' can be calibratedso as to reduce the effects of detuning of the antenna circuits bymineralized layers around the drillholes 18 or 18'. Third, the effectsof surface wave modulation can be cancelled. Fourth, the transmittingand receiving antennas can be located in the same borehole. Fifth, thegeological noise, caused for example by changes in the electricalparameters of ore bodies, can be subtracted from the received signal.Sixth, a fast rise time digital signal is used in place of a sinusoidalsignal to minimize phase jitter in the downhole receiver and transmitterPLL circuits. The six categories of modifications are illustrated inFIGS. 13-20.

FIG. 13 shows a receiver antenna 202 positioned orthogonally to thetransmitting antenna 104'. Elements in FIG. 13 that are identical toelements shown in FIGS. 1 or 6 are designated by the same numeralfollowed by a prime symbol.

The antenna 104' is a vertical magnetic dipole antenna, such as aferrite rod antenna, with the loop of the antenna orientated in the x-zplane. For a ferrite rod antenna, the ferrite rod would run parallel tothe drillhole 18' (i.e. parallel to the y axis) and the loops of thewire coil wound around the ferrite rod would lie in the x-z plane.

The receiver antenna 202 is a horizontal magnetic dipole antenna havingthe antenna loop lying in the y-z plane. Thus, the loops of the antennas104' and 202 are orthogonal to each other. This orthogonal orientationhas the effect of cancelling or suppressing reception at the receiverantenna 202 of a primary wave 206 emitted by the transmitting antenna104'.

The electromotive force (emf) induced in the receiver antenna 202 isgiven by equation (3), supra.

The magnetic field components for the primary wave 206 are given byequations (4) and (5) discussed in relation to FIG. 7. Since neitherH₁₀₀ nor H_(r) thread the loop of the receiver antenna 202 by theprimary wave 206. In contrast, a scattered wave 210, produced by theconductor 14', is a cylindrical spreading wave that has a magnetic fieldcomponent H_(s) that threads the loop of the receiver antenna 202. Thus,the scattered wave 210 induces emf in the receiver antenna 202, allowingdetection of the scattered wave 210.

The use of orthogonal transmitting and receiving antennas to allowreception of the scattered wave 202 while discriminating against theprimary wave 206 is used, preferably, with methods III and V discussedin relation to FIG. 8 and FIG. 10.

FIG. 14 shows that the orthogonal antenna arrangement of FIG. 13 canalso be used to detect a conducting surface 218 such as a rock mass(i.e. a "stringer" or a "skan") or a coal seam. Typically, the rock massis a magmatic sulfide injection in the host rock. A primary wave 220,emitted by the transmitter antenna 104', is reflected off the conductingsurface 218 as a scattered wave 222. Because the plane containing theloop of the transmitter antenna 104', e.g. the coils of a ferrite rodantenna) are orthogonal to the plane containing the loop of the receiverantenna 202, only the magnetic field component of the scattered wave 222threads the loop of receiver antenna 202. Therefore, only the scatteredwave 222 is detected.

FIG. 15 shows a block diagram of a calibrated underground conductordetection system 230 comprising the frequency source unit 32', adownhole transmitter 234 and a downhole receiver 238. The system 230 isdesigned for use with methods II, III or V illustrated in FIGS. 6, 8 and10, respectively. The downhole transmitter 234 replaces the transmitter100 (or 100') and the downhole receiver 238 replaces the receiver 36 (or36').

The frequency source unit 32' directs a synchronized signal S_(syn) (2.5kHz) to a fiber optic (F.O.) transmitter 242 which converts S_(syn) to adigital waveform (instead of sinusoidal) and is used to modulate thelight intensity of a signal transmitted downhole via a fiber optic cable244 to a fiber optic receiver 246. The output of the receiver 246 isdirected to a phase locked loop (PLL) circuit 250 (with a multiplicationconstant M) whose output is directed to a transmitter 252. The output ofthe transmitter 252 (MS_(syn)) is applied to a tuned vertical magneticdipole antenna 254 (analogous to antenna 104) such as a ferrite rodantenna. Current flowing through a sensing resistor 256, connected tothe antenna 254, produces a voltage e_(a) which is converted to a D.C.voltage that is used to control the magnitude of the signal emitted bythe transmitter 252 by means of a feedback system 258.

The feedback system 258 comprises an amplitude detector 260, a phasedetector 262, an amplifier output level control circuit 264, amicrocomputer 266 and an analog-to-digital (A/D) converter 268. Thephase detector 262 compares the output phase of the PLL circuit 250 withthe phase of the current (I) flowing through resistor 256. The amplitudedetector 260 and the level control circuit 264 insure that the current Iis maintained at a constant level. The measured value and phase ofcurrent I are converted (encoded) to a digital format signal and sent toa fiber optic transmitter 270 and then up a fiber optic cable 271 to afiber optic receiver 272. The output of the F.O. receiver is directed tothe frequency source control unit 32' where the digital format signal isdecoded. The microcomputer 72 operates an LCD display to indicate themagnitude and phase of the antenna current for calibration purposes.

The downhole receiver 238 functions analogously to the receiver 36' toreceive the magnetic field component H.sub.φ of EM₂ as previouslydescribed with respect to methods I-IV. However, the downhole receiver238 includes a calibration antenna 274 which may be an untuned broadbandcircuit such as a long wire or an untuned loop or rod antenna.

A synchronized signal S_(syn), having a frequency f_(syn) (e.g. 10 kHz)is generated by the frequency source unit 32', converted to a fast risetime digital signal and applied to a fiber optic transmitter 276. Thedigital signal S_(syn) is sent downhole via a fiber optic cable 277 to afiber optic receiver 278. After processing in a state machine 280, thesignal S_(syn) (2.5 kHz) is applied to a PLL circuit 281, with an "M"multiplication constant, to yield a signal Mf_(syn) which has the samefrequency as the downhole transmitter 234. The signal Mf_(syn) isapplied to the calibrating antenna 274 which radiates the signalMf_(syn) to a receiving antenna 283. The receiving antenna 283 is atuned magnetic dipole antenna, analogous to the antenna 41 (and 41'),and also receives the signal EM₂ radiated by the conductor 14'.

The 10 kHz signal f_(syn) is also directed to a PLL circuit 285 where itis multiplied by a constant factor N yielding a signal Nf_(syn). Thesignal Mf_(syn), as received at antenna 283 and the signal Nf_(syn) aremixed in a mixer 287 to produce a mixer output signal called anintermediate frequency (IF) signal at the frequency S_(syn). Anamplifier 289 and an attenuator 291 are positioned between the mixer 287and the antenna 283. The IF signal S_(syn) is sent uphole to thefrequency source unit 32' via the pathway that includes a filter 292, anIF amplifier 293, a fiber optic transmitter 294, a fiber optic cable 295and a filter optic receiver 296. A microcomputer 297 and ananalog-to-digital (A/D) converter 298 are connected between the F.O.receiver 278 and the F.O. transmitter 294. The A/D converter 298 is usedto measure the magnitude of the IF signal and, via the microcomputer297, automatically adjusts the attentuator 291.

Use of the downhole receiver 238 allows the source unit 32' to establishthe magnitude and phase of the receiver transfer function from theantenna 283 to the frequency source unit 32'.

The functioning of the calibrated underground conductor detection system230 is as follows. As the antennas 254 and 283 descend in the drillhole18', mineralized layers such as brine or sulfide ore zones detune theantenna circuits. The detuning causes changes in the current I flowingthrough the antenna 254 (and resistor 256) to occur.

The calibration system 230 permits the downhole transmitter 234 and thedownhole receiver 238 to be calibrated to compensate for these detuningeffects by the mineralized layers. The location of the mineralizedlayers can also be mapped by measuring the depth of the instrumentswhere detuning occurs.

FIG. 16 shows a receiving system configuration, represented by thegeneral reference numeral 300, for using a surface electromagneticsignal 302 to detect the buried conductor 14'. The surfaceelectromagnetic wave 302 could be generated, for example, by a lightningdischarge, a radio broadcast transmitter (e.g. the 17.4 kHz transmitterin Japan), a controlled source audio frequency magnetotelluric (CSAMT)transmission, or a high power navigation transmitter.

The system 300 illustrated in FIG. 16 is similar to the system shown inFIG. 9. Elements in FIG. 16 that are identical to elements in FIG. 9 arerepresented by the same numeral.

A surface receiver 303 and an antenna 304 are utilized to receive thesignal 302 propagating in the earth ionosphere waveguide. For example,there is a large radio transmitter in Japan that transmits a 17.4 kHzsignal that is detectable in Korea. The receiver 303 is connected to thesource unit 32' by a fiber optic cable 305. The received signal 302 ismixed in the receiver 303 with the 2.5 kHz synchronization signalgenerated by the frequency source unit 32' to yield a signal f_(If) '.The signal f_(If) ' is sent to a downhole receiver 306 via a fiber opticcable 308. The fiber optic cable 308 is nonconductive preventing thesignal 302 from propagating down the cable to the downhole receiver 306.It should be noted that systems using conductive wire line cable tolower instrumentation into boreholes cannot be successfully used intunnel detection. The conductive wire can transmit radio signals ormagnetotelluric signals to the downhole receiving antenna. Similarly,signals such as power line harmonic signals can also propagate down thecable to the downhole receiver 306. These unwanted waves and signalsinterfere with the tunnel location process.

The downhole receiver 306 includes a receiving antenna 312 for receivingthe radiated waves from the conductor 14' as described in relation toFIG. 9 and method IV.

FIG. 17 illustrates the electrical components of the surface receiver303 and the downhole receiver 306 in more detail. The antenna 304 isconnected to an attenuator 314 which is connected to a radio frequency(RF) amplifier 316 which is connected to a mixer 318. The 2.5 kHzsynchronized signal from the frequency source unit 32' is inputted intothe mixer 318. The output of the mixer 318 is filtered through abandpass filter 320 and amplified by an IF amplifier 322. Amicrocomputer 324 is connected to the attenuator 314 and to thefrequency source unit 32'. The output from the IF amplifier 322 isdirected to a fiber optic transmitter 328 and the resulting light outputis directed down the fiber optic cable 308 to the downhole receiver 306.

In operation, the mixer 318 always transposes the signal received by theantenna 292 to an intermediate frequency (IF) that is 2.5 kHz below thereceived signal frequency. The IF signal is then amplified (limited) bythe amplifier 322 and converted to a digital signal that modulates thelight intensity of the fiber optic transmitter 328. The bandpass filter320 must be returned to each IF signal frequency. The microcomputer 324automatically adjusts the attenuator 314 in response to the IF signallevel at the F.O. transmitter 328. The microcomputer 324 also usesserial data transmission to communicate with the frequency source unit32'.

The downhole receiver 306 includes a fiber optic receiver (alight-to-analog converter) 332 whose output is directed to a mixer 334.The antenna 312 is connected to an RF amplifier 336 whose output isdirected to the mixer 334. The output of the mixer 334 is filteredthrough a filter 338 and amplified by an IF amplifier 340. The output ofthe IF amplifier is directed to a fiber optic transmitter 342 whoselight output is sent up the fiber optic cable 37' to a fiber opticreceiver 344 and then to the frequency source unit 32'.

The method of using the system 300 can now be explained. The surfacesignal 302 produces a received signal S_(R) at the antenna 304represented by equation (17).

    S.sub.R =A sin [2πf.sub.o t+χ.sub.1 (t)]            (17)

where

A=the magnitude of signal 302;

f_(o) =the frequency of signal 302; and

χ₁ (t)=the phase of signal 302 which may be modulated.

The source unit 32' produces a synchronization signal S_(syn) of theform represented by equation (18):

    S.sub.syn =B sin (2πf.sub.syn t+χ.sub.2)            (18)

where

f_(syn) =the frequency of S_(syn) ; and

χ₂ =phase of S_(syn).

The mixer 318 mixes S_(R) with S_(syn) to yield an intermediate signal(S_(IF)) represented by equation (19):

    S.sub.IF =C sin (2π[f.sub.o -f.sub.syn ]t+0λt)-0.sub.2)(19)

The signal S_(IF) is applied to the bandpass filter 320 which suppressesthe S_(syn) and S_(R) signals. The filtered S_(IF) signal is applied tothe F.O. transmitter 328 for light intensity modulation transmission tothe mixer 334.

The surface electromagnetic signal 302 induces current flow in theconductor 14' in the following manner. The surface signal 302 istransmitted long distances by the waveguide formed by the surface of theearth and the ionosphere. The surface signal 302 has a vertical electricfield component E_(z) that is vertically polarized in the waveguide anda horizontal electric field component E_(m) and a horizontal magneticfield component H_(y) that are horizontally polarized in the waveguide.

The horizontal components E_(m) and H_(y) propagate into the earth wherethe electric field component E_(m) can couple with the conductor 14' andinduce current flow in the conductor 14'. The current flow in theconductor 14' initiates secondary wave propagation in the rock layer 12'which is detected by the antenna 312 as the signal S_(m) represented byequation (20):

    S.sub.m =D sin [2πf.sub.o t+χ.sub.m +χ.sub.1 (t)](20)

where

χ_(m) =the phase shift accumulated by signal 302 as it propagates fromthe surface, along the conductor 14' and through rock layer 12' toantenna 312.

In the mixer 334, the signals S_(IF) and S_(m) are mixed so as to cancelout the term χ₁ (t) leaving a second IF signal S_(IF2) given by equation(21):

    S.sub.IF2 =E sin (2πf.sub.syn t+χ.sub.m +χ.sub.2)(21)

The signal S_(IF2) is amplified and sent up the fiber optic cable 37' tothe frequency source unit 32' where the magnitude and phase of theS_(IF2) signal is determined by the synchronous (autocorrelation)techniques described previously with respect to FIG. 5.

The reason for using the receiving system configuration 300 is that inorder to achieve the maximum radius of detection for the conductor 14',the sensitivity of the downhole receiver 306 must be maximized.Ordinarily, this is difficult to achieve because the bandwidth of thereceiver 306 must be wide enough to accommodate the occupied bandwidthof the terrestrial radio signal 302. The surface receiver 303 allows afrequency transposition scheme to be utilized which allows the bandwidthof the downhole receiver 306 to be minimized. In summary, the frequencytransposition scheme converts the signal received at antenna 312 to acontinuous wave (CW) signal that is processed using real time optimumsynchronous (autocorrelation) techniques. The advantage of the bandwidthcompression obtained by this method is that the noise bandwidth of thedownhole receiver is reduced to less than 1 Hz and the receivingsensitivity is less than one nanovolt.

FIG. 18 shows another embodiment of the orthogonal antenna arrangementshown in FIGS. 13 and 14. In FIG. 18, a dual antenna 348 comprising thetransmitter antenna 104' and the receiver antenna 202, is located in thedrillhole 18'. A motor 350 is connected between the antenna 104' and theantenna 202 for causing the antenna 202 to rotate about the drillholecenterline 354.

The antenna 104' is a ferrite rod antenna, while the antenna 202 is anelongated loop antenna. The antenna 202 is rotated so that it willreceive the scattered wave or reflected wave (EM₂) emitted from theconductor 14' or conducting surface 218. The transmitter 100' and thereceiver 36' are also both located in the drillhole 18' with the dualantenna 348. The dual antenna 348 is of use in applications where, forexample, the conductor 14' is an ore deposit or a fault in an oilreservoir.

FIG. 19 illustrates a method for detecting an anomalous geological zone360. The anomalous geological zone 360 is defined as a region within therock layer 12' that has different electrical properties than the rocklayer 12' (e.g. a tunnel). For convenience, this method is referred toas method VII. Elements of FIG. 19 that are identical to elementsdescribed previously are designated by the same reference numeralfollowed by a prime symbol. Method VII takes advantage of the fact thata low frequency (30-300 kHz) or a medium frequency (300 kHz to 3 MHz)scan of the anomalous geological zone 360 will map conductivity changesdue to geologic changes in the rock layer 12'. A second scan done athigh frequency (3-30 MHz), very high frequency (30-300 MHz) or ultrahigh frequency will yield a tomography scan that includes both thegeologic background conductivity changes and a diffraction shadow due tothe geological zone 360. The geologic noise can then be eliminated fromthe tomography scan by subtracting the LF/MF scan data from the HF/VHFscan data leaving a tomographic image of the diffraction shadow due tothe anomalous geological zone 360.

Typically, in method VII a shadow scattering wave scan technique in theHF/VHF band (described previously with respect to FIG. 8) is used. Inmethod VII, the relative position of the transmitter 100' versus theposition of the receiver 36' is varied. For example, the transmitter100' is positioned in the first drillhole 18' at a position P₁corresponding to a depth d₁. The receiver 36' is positioned in thesecond drillhole 18" at a position P_(1R) corresponding to the depth d₁.A first LF or MF signal is then transmitted from the transmitter 100' tothe receiver 36' and conveyed up the fiber optic cable 37' forprocessing in the source unit 32'.

The transmitter 100' is then moved to a position P₂ corresponding to adepth d₂ and a second LF or MF signal is transmitted from thetransmitter 100' to the receiver 36' which is still positioned atP_(1R). This procedure is repeated until the transmitter 100' has beenpositioned at each of the P_(n) transmitter positions (generallyn=10-25). The receiver 36' is then moved to a second receiver positionP_(2R), corresponding to the depth d₂, and the entire scanning processis repeated by moving the transmitter 100' to each of the P_(n)transmitter positions while th receiver 36' remains fixed at thereceiver position P_(2R). The process is continued until the receiver36' has been moved to each of the P_(nR) receiver positions. Thisprocess generates a plurality of ray paths 364 where the number of raypaths 364 is equal to n².

The entire scanning process is then repeated using an HF/VHF transmitterand receiver to generate an HF/VHF data set. The HF/VHF data set isgenerated by positioning the HF/VHF transmitter and receiver at each ofthe positions P_(n) or P_(nR), respectively, and transmitting HF/VHFsignal over each of the n² ray paths 364.

In the tomographic process, the image plane between the drillholes 18'and 18" is divided into a plurality of cells or pixels. For example,horizontal lines are drawn from points midway between the P_(n)transmitter positions. Equally spaced vertical lines are then drawnperpendicular to the horizontal lines to form the plurality of pixels.Since the ray path distance through each pixel is known, an averageparameter value, such as the attenuation rate or phase shift, in eachpixel can be determined by processing measured data in an iterativeimage reconstruction algorithm. The image of the anomalous zone 360 isgraphically reconstructed by contouring the pixel attenuation rate andphase values.

In the LF and MF bands, conduction currents predominate in the rockmass. The attenuation rate and phase constants are proportional to thehalf power of the electrical conductivity of the rock mass and thefrequency of the wave. CW tomography images map contours of constantattenuation rate or phase across the image plane. The general shape ofthe anomalous geologic zone 360 appears in the contouring of thereconstructed pixel values. The change in the primary wave depends uponthe path length and the change in conductivity in the rock mass.

In the LF and MF bands, the wavelength in the rock mass is oftentimesgreater than the anomalous zone. The high attenuation rate and lowscattering cross section combine to create negligible secondary waves.The scattering Fresnel ellipsoid surface (radiating and receivingantenna located at the foci points) resemble narrow pencil beams inslightly conducting rock mass. The narrow beam gives rise to the raypath notion used in the iterative reconstruction of RIM tomographicimages.

In the HF, VHF, and UHF bands, displacement currents predominate in therock mass. The contours of constant attenuation rate map changes in thefirst power of conductivity and inverse half power of the dielectricconstant across the image plane. The attenuation rate would increasefrom MF band values to greater values in the HF, VHF, and UHF bands. Theincrease in attenuation rate can be analytically determined. Bysubtracting the MF tomographic image of "geologic" noise from the HF,VHF, or UHF images on a pixel by pixel basis, the deterministic"geologic" noise can be substantially removed from the HF, VHF, or UHFimage. Thus, in method VII, a typical scanning protocol would be asfollows:

1. The receiver 36' is positioned at depth d₁ ;

2. The transmitter 100' is positioned at a plurality of transmitterstations corresponding to the depths d₁ through d_(n) (i.e. at as manypositions as are required by the scan. Ten or more points may bemeasured);

3. At each transmitter station an LF or MF signal is transmitted to thereceiver 36' along one of the ray paths 364;

4. The receiver 36' is moved to a new depth (receiver station) and steps2 and 3 are repeated;

5. Step 4 is repeated until all the depths required by the tomographyscan have been sampled;

6. Steps 1-5 are repeated substituting an HF, VHF or UHF signal for theLF/MF signal in step 3; and

7. The data for the LF/MF and the HF/VHF scan are processed in atomography algorithm. The LF/MF image values are subtracted from theHF/VHF image values on a pixel by pixel basis to remove the geologicnoise and enhance the anomalous geological zone diffraction pattern.

In the preferred embodiment of method VII, a 522.5 kHz signal is used asthe MF signal and a 15 to 30 MHz signal is used as the HF signal. Also,the receiver antenna 41' may be a vertical magnetic dipole or positionedlike the antenna 202 in FIGS. 13 and 14 so as to suppress reception ofthe primary wave.

The measured data set is processed by an iterative reconstructionalgorithm. Dines and Lytle (1979) describe an algorithm that can be usedto process the data. Since the Kth ray path distance (d_(ijk)) througheach pixel is known, the average attenuation rate (α_(ij)) in each pixelcan be determined from the total loss (I_(Lk) i_(k)) measured on the Kthpath as: ##EQU9##

The iterative process solves K linear equation in IJ unknowns. Theaverage value of α_(ij) is determined from the set of measured path lossvalues L_(k). The summation in the above equation is over all values ofI and J where d_(ijk) =0 in pixels not intersected by the Kth ray path.Because the length and width limitations in a scan, the number ofequations is usually insufficient to determine α_(ij) uniquely. And as aresult, the set of linear equations is over or under determined. Insteadof direct matrix conversion, iterative solutions are used to determineα_(ij). The Algebraic Reconstruction Technique (ART) treats one equationat a time. ART changes the pixel values found processing each ray pathdata. Simultaneous Iterative Reconstruction Techniques (SIRT) change thepixels after processing all paths.

The iterative process begins with an initial guess of the pixel values(conductivity, attentuation rate, or phase shift). Multiple iterationsroutinely modify the pixel values until the ray path signalssynthetically determined from the pixels is within a few percent of themeasured data set. The set of pixel values represents the model of thegeologic disturbance zone. A contouring algorithm is used to generatecontours of constant pixel value curves across the image plane.

Methods of improving the reconstruction methods have been found and someare under development. The construction routines specify the systemcoupling factor 20 Log C'. See L. Stolarczyk, U.S. Pat. No. 4,742,305.The 20 Log C' factor and spherical spreading factor are used todetermine the path loss (L_(k)). A transmitter-fan ray path data can begraphically constructed to find 20 Log C' and α_(ij). If 20 Log C' isselected to be too high, then the α_(ij) pixel values will beoverstated. Oftentimes the conductivity along the drillhole is known andcan be included in the vertical edge pixel values. Weighting longer raypaths has also improved the reliability of the reconstructions in somecases.

FIG. 20 shows a fast rise time digital signal waveform 370 used totransfer the synchronization signals to the downhole instruments.Electrical noise adds to the synchronization signals and causesexcessive phase jitter in the downhole PLL's 140, 250, 285 and 281 whenthe synchronization signal has a sinusoidal waveform. The phase jitterproblem is minimized when fast rise time synchronization signals areused in the system.

The fast rise time digital signal waveform 360 is used to modulate thelight intensity transmitted from a fiber optic transmitter such as theF.O. transmitters 242, 276 or 303.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter having read the above disclosure. Accordingly, it is intended thatthe appended claims be interpreted as covering all alterations andmodifications as fall within the true spirit and scope of the invention.

I claim:
 1. A method for calibrating a downhole receiver used fordetecting an underground conductor comprising:a. generating asynchronization signal at a signal generator located at a surfaceposition; b. causing said synchronization signal to be sent over a firstfiber optic cable from said signal generator to a receiver located in adrillhole, the receiver having a tuned loop magnetic dipole antenna forreceiving a scattered wave generated by an underground conductor, andmeans for directing a signal pathway extending from said tuned loopmagnetic dipole antenna to said signal generator; c. causing acalibration antenna in said receiver to transmit a first calibrationsignal, related to said synchronization signal, to said tuned loopmagnetic dipole antenna; d. causing a second signal related to saidfirst calibration signal to be sent from said tuned loop magnetic dipoleantenna over said signal pathway to said signal generator; and e.calculating at said signal generator a phase or magnitude parameter ofsaid second signal used to calibrate said receiver.
 2. The method ofclaim 1 wherein, the synchronization signal comprises a fast rise timedigital waveform.